Terephthalic acid is used in large quantities as a starting material for the preparation of polyester fibers such as linear polyalkylene terephthalates. These polyesters can be prepared quite conveniently by, for example, direct condensation of ethylene glycol with terephthalic acid. In such processes the terephthalic acid used in fiber production must be of exceptionally high purity. However, terephthalic acid prepared by, for example, the oxidation of p-xylene contains oxidation intermediates as impurities. These intermediates must be removed from the acid to obtain a material suitable for use in making polyester fibers. A number of methods have been employed for purifying terephthalic acid by chemical or physical treatment to obtain an acid suitable for fiber production. For example, Canadian Pat. No. 768,189 discloses isolating pure terephthalic acid from an oxidation mixture by heating the oxidate at 210.degree. to 280.degree. C. for about 1/2 to 5 hours and recovering the purified terephthalic acid. Furthermore, p-xylene has been used to remove impurities from terephthalic acid but these methods have been digestion procedures with all their attentant disadvantages.
Other references of interest include U.S. Pat. Nos. 3,624,145; 3,546,285; 3,584,039 and 3,850,983. None of these references relate to the removal of the para-toluic acid with paraxylene while the terephthalic acid is in a water slurry during the crystallization step in the process for purifying terephthalic acid by hydrogenation under pressure in the presence of a metal catalyst. Most of the prior art processes are directed to the removal of 4-carboxybenzaldehyde and not to the removal of para-toluic acid.
Process of the invention is conducted at elevated temperature and pressure while the terephthalic acid is dissolved in water. By reason of its low solubility in water, terephthalic acid requires either large volumes of water or high temperatures in order for the desired terephthalic acid production quantity to be put into solution. For reasons of economic equipment design and process operation, it is therefore desirable to conduct the process within the range of about 392.degree. to about 700.degree. F., although lower or higher temperatures may be used in particular circumstances. The most advantageous temperature range is about 440.degree.-575.degree. F., e.g., 464.degree.-550.degree. F. The quantity of water needed to dissolve the terephthalic acid at various temperatures may be estimated from the table below:
TABLE 1 ______________________________________ Terephthalic acid, Temperature, .degree.F. g/100 g, H.sub.2 O for solution ______________________________________ 1 365 5 401 10 468 20 498 30 522 ______________________________________
For the purification of crude terephthalic acid, aqueous solution temperatures in the range of 450.degree. to 600.degree. F. are preferred because these solutions carry more than 5 pounds of the acid per 100 pounds of water.
Pressure conditions for the process depend upon the temperature at which this process is conducted. Since the temperature at which significant amounts of the impure terephthalic acid may be dissolved in water are substantially above the normal boiling point of water, and since the hydrogenation section is to be carried out with the solvent in the liquid phase, the pressure will necessarily be substantially above atmospheric pressure.
It is advantageous to trickle the liquid solution of the terephthalic acid through a bed of the catalyst because lower hydrogen partial pressure or hydrogen driving force is required than is required when the catalyst bed is operated liquid full. Either a static hydrogen atmosphere or a flow, concurrent or countercurrent, of hydrogen through the catalyst chamber may be maintained. Lower hydrogen partial pressures are required for the trickle or percolation method of conducting the hydrogenation section because there is provided a thin film of the aqueous solution of the impure terephthalic acid on the catalyst particles and thus a lower hydrogen driving force is needed for the hydrogen to dissolve and diffuse through the thin liquid layer and reach the catalyst. For such percolation method of conducting the hydrogenation a continuous flow or atmosphere of hydrogen is not essential. However, for maximum hydrogenation rates it is beneficial to dissolve at least some hydrogen into the solution, conveniently in the acid dissolver, prior to contacting it with the catalyst. Hydrogen may be intermittently introduced into the bed of extended catalyst during the continuous introduction of the aqueous solution of impure terephthalic acid. The minimum of hydrogen to be introduced intermittently is, of course, an amount of hydrogen in excess of that required for reduction of the dissolved impurity so that adsorption of the excess hydrogen in the porous catalyst support can be simultaneously accomplished. Very little hydrogen is consumed by the purification process.
It is particularly advantageous to impose on the aqueous solution being treated a pressure above the pressure required to maintain a liquid phase of the aqueous solution of impure terephthalic acid and dissolved hydrogen. This additional pressure prevents premature crystallization of the acid due to minor process pressure variations causing vaporization of some of the solvent. This is readily accomplished by use of an inert, non-condensable gas, such as nitrogen. By "inert" gas is meant that gas which is not reactive with the terephthalic acid or the hydrogen or solvent. Nitrogen is a convenient inert gas. An additional benefit accruing from the use of nitrogen is that the dilution of the hydrogen introduced into the process provides low partial pressure of hydrogen to minimize over-hydrogenaton, such as, for example, saturation of aromatic nuclei.
Hydrogen treating time, or space velocity, will depend on the initial terephthalic acid purity, that is, the amount of impurity to be reduced, on the desired fiber-grade specifications imposed on the purified terephthalic acid, and on other conditions of the hydrogenation, such as, for example, catalyst activity. Ordinarily, a treating time, i.e. contact time with the catalyst, within the range of about 0.001 to about 10 hours, advantageously about 0.01 to 2 hours, will suffice for most operations. Although treating time is not a critical variable, it must be taken into consideration with regard to the aforementioned severe hydrogenation and its side effects.
The hydrogenation catalyst required, for the process of this invention, to convert the aldehyde carbonyl group on the 4-carboxybenzaldehyde (4-CBA) to para-toluic acid and to destroy, or otherwise render innocuous, other impurities present (e.g. those of benzil and fluorenone structure) in the feed terephthalic acid are preferably a Group VIII noble metal, preferably platinum and/or palladium, supported on adsorbent, high surface area charcoal. A wide variety of catalysts have been found efficacious, and while carbon-supported noble metals are outstanding, reference may be made to any of the standard texts on hydrogenation or catalysts for alternative materials which are catalytically effective under aqueous phase hydrogenation conditions. It must be kept in mind, however, that the catalyst used must be one which is useful for effecting the hydrogenation under mild hydrogenation conditions as defined herein. Numerous catalysts are listed, for example, in Kirk and Othmer's Encyclopedia of Chemical Technology (Interscience), particularly the chapters on Hydrogenation and Catalysts; Emmett's Catalysis, (Reinhold), particularly Volumes IV and I on Hydrogenaton; Lohse's Catalytic Chemistry (Chemical Publishing Company), particularly the sections on Group VIII metal Catalysts; and such U.S. patents as U.S. Pat. Nos. 2,070,770 and 2,105,664. Illustrative catalysts include the Group VIII Nobel Metals Ruthenium, Rhodium, Palladium, Osmium, Iridium, and Platinum, advantageously extended on a support, such as activated carbon, e.g., adsorbent charcoal. Advantageously, noble metal contents in the range of about 0.05-0.5 weight percent may be used, with about 0.1-0.3 weight percent being the preferred noble metal content for use in trickle beds of catalyst. The higher noble metal contents tend to produce over-hydrogenation while the lesser amounts suffer some loss in hydrogenation activity as compared with catalysts of the preferred noble metal content.
The adsorbent charcoal support for the noble metal may be any such support which has sufficient mechanical strength and surface area. It has been found that palladium-charcoal catalysts having a palladium content in the preferred range of 0.1-0.3 weight percent and also having a very high surface area in the range of about 1000-3000 square meters per gram of catalyst are particularly well suited for use in the present invention.
The hydrogen treated solution is filtered to remove any suspended solids, such as catalyst support fines and extraneous materials, of about 5 microns and larger in size. The purified acid is then recovered from the filtered solution. Crystallization is a convenient and preferred method for recovering the purified acid. Either batch or continuous crystallization may be employed in the crystallization section. Crystallized acid is recovered by centrifuging, during which further purification is effected by washing the centrifuge cake. The crystals are dried in a rotary kiln to a moisture content below about 1 wt. percent, preferably about 0.02-0.06 wt. percent, to prevent caking during subsequent storage and shipping.
Purified terephthalic acid obtained from the oxidation of p-xylene with molecular oxygen and reduction by hydrogenation contains certain impurities, primarily para-toluic acid, and amounts of other partial oxidation products that are also extremely difficult to remove. According to the present invention, a solution of purified terephthalic acid containing a weight ratio of acid to water of from about 5:100 to about 1:1, and preferably from about 1:5 to about 1:1, is mixed with a paraxylene liquid thereby forming a two-phase system of an aqueous phase containing purified terephthalic acid and an organic phase containing para-toluic acid and paraxylene. The paraxylene is added in an amount to produce a ratio of aqueous phase to organic phase of from about 1:1 to about 50:1, preferably from about 6:1 to about 10:1, separating the aqueous phase from the organic phase containing para-toluic acid, from the aqueous phase containing purified terephthalic acid, and then recovering purified terephthalic acid from the aqueous phase and recycling the para-toluic acid and paraxylene to the paraxylene oxidation reactor. Important features for the successful operation of this process are the weight ratio of purified terephthalic acid to water, the ratio of aqueous to organic phase, and the temperature maintained during extraction of the impurities. The process must be conducted under such conditions that impurities are extracted from an aqueous solution of terephthalic acid as contrasted to removal of impurities from solid terephthalic acid, i.e., digestion procedures.
The primary constituent of the organic phase that solvates the impurities in the liquid-liquid extraction process is paraxylene. The preferred extract is paraxylene since it can readily be recycled to the paraxylene oxidation reactor with para-toluic acid. The water-immiscible liquid paraxylene and the aqueous solution of terephthalic acid, when mixed in a single vessel in a batch process, or a series of mixers arranged to effect a countercurrent type extraction, or a continuous countercurrent flow through a single column, form an aqueous phase and an organic phase at the temperature stated above. Initially, the aqueous phase contains primarily terephthalic acid and impurities, whereas the organic phase contains primarily the paraxylene that is inert to terephthalic acid. However, after mixing the aqueous phase and the organic phase, the paraxylene solvates and removes para-toluic acid from the aqueous solution of terephthalic acid. The amount of time for the paraxylene treatment depends upon the particular procedure used and, of course, the amounts of ingredients treated. In general, a multiple stage batch process, in which the organic phase and aqueous phase are mixed by mechanical agitation and allowed to settle, or a continuous process employing a series of mixers and settlers of the type used in concurrent or countercurrent extraction systems, requires about 1 to about 10 minutes, usually about 4 to about 5 minutes, from mixing until extraction is complete, purified terephthalic acid recovered and para-toluic acid paraxylene mixture is recycled.
After paraxylene has solvated and removed the para-toluic acid from the aqueous solution of purified terephthalic acid, the organic phase contains paraxylene and para-toluic acid and the aqueous phase containing terephthalic acid is segregated, the organic phase is directed to the oxidation reactor, and the aqueous phase directed to the remaining crystallization or, alternatively, to centrifuges.